Hydrogen Absorbing Alloy, Hydrogen Absorbing Alloy Electrode, Secondary Battery and Production Method of Hydrogen Absorbing Alloy

ABSTRACT

The present invention provides a hydrogen absorbing alloy containing a phase of a Pr 5 Co 19  type crystal structure having a composition defined by a general formula A (4−w) B (1+w) C 19  (A denotes one or more element(s) selected from rare earth elements including Y (yttrium); B denotes an Mg element; C denotes one or more element(s) selected from a group consisting of Ni, Co, Mn, and Al; and w denotes a numeral in a range from −0.1 to 0.8) and having a composition as a whole defined by a general formula R1 x R2 y R3 z  (15.8≦x≦17.8, 3.4≦y≦5.0, 78.8≦z≦79.6, and x+y+z=100; R1 denotes one or more element(s) selected from rare earth elements including Y (yttrium); R2 denotes an Mg element, R3 denotes one or more element(s) selected from a group consisting of Ni, Co, Mn, and Al; the numeral of Mn+Al in the z is 0.5 or higher; and the numeral of Al in the z is 4.1 or lower).

TECHNICAL FIELD

The present invention relates to a hydrogen absorbing alloy, a hydrogenabsorbing alloy electrode, a secondary battery, and a production methodof a hydrogen absorbing alloy.

BACKGROUND ART

A hydrogen absorbing alloy is an alloy capable of safely and easilystoring hydrogen as an energy source and therefore the alloy has drawnlots of attention as a new material for energy conversion and storage.

Application fields of the hydrogen absorbing alloy as a functionalmaterial have been proposed in a wide range such as storage andtransportation of hydrogen, storage and transportation of heat,heat-mechanical energy conversion, separation and refining of hydrogen,separation of hydrogen isotopes, batteries using hydrogen as an activemass, catalysts for synthetic chemistry, and temperature sensors.

For instance, a nickel-hydrogen storage battery using a hydrogenabsorbing alloy as a negative electrode material has followingcharacteristics; (a) having a high capacity; (b) highly durable toovercharge and overdischarge; (c) capable of charging and discharging athigh efficiency; and (d) is clean and therefore, the battery has drawnattention as a consumer battery and to further improve functions andcapabilities (e.g. to improve charging and discharging cyclecharacteristics and capacities of batteries), its applications andpractical uses have been actively promoted.

As an electrode material for a nickel-hydrogen storage battery, which isone application example of such a hydrogen absorbing alloy, arepractically used AB₅ type rare earth-Ni based alloys having a CaCu₅ typecrystal structure; however, the discharge capacity of the alloy islimited to about 300 mAh/g and it is difficult to further increase thecapacity in the present state.

On the other hand, in recent years, various kinds of rare earth-Mg—Nibased alloys have drawn attention as new hydrogen absorbing alloysprovided with durability which AB₅ based hydrogen absorbing alloys haveand a high capacity which AB₂ based hydrogen absorbing alloys have incombination and it is reported that use of the alloys as an electrodemakes it possible to have a discharge capacity exceeding that achievedby using an AB₅ type alloy.

For instance, the following Patent Document 1 discloses electrodescontaining LaCaMgNi₉ alloys having a PuNi₃ type crystal structure.

Patent Document 1: Japanese Patent No. 3015885

However, although having large hydrogen absorption capacities, thealloys described in Patent Document 1 have a problem that the alloyshave low hydrogen releasing speeds (in other words, being inferior inthe rate characteristics).

Further, Patent Document 2 discloses hydrogen absorbing alloyscontaining a phase of intermetallic compounds defined as La₅Ni₁₉ andadditionally hydrogen absorbing alloys containing a phase ofintermetallic compounds defined as (La-M)₅Ni₁₉(M:Ca, Mg). The hydrogenabsorbing alloys described in Patent Document 2 are produced bymechanical alloying using two or more types of different hydrogenabsorbing alloys as the material.

Patent Document 2: Japanese Patent No. 3397981

Further, the following Patent Documents 3 to 5 disclose that electrodesusing rare earth-Mg—Ni based alloys having crystal structures such as aCeNi₃ type, a Gd₂Co₇ type, a Ce₂Ni₇ type, and a PuNi₃ type show goodhydrogen releasing characteristics while keeping high hydrogen storagecapacities.

Patent Document 3: Japanese Patent Application Laid-Open (JP-A) No.11-323469

Patent Document 4: JP-A No. 2002-273346

Patent Document 5: JP-A No. 2002-105563

Further, the following Patent Document 6 discloses that with respect toalloys having a Ce₅Co₁₉ type crystal structure, electrodes producedusing the alloys compounded with rare earth-Ni alloys having a CaCu₅type crystal structure are excellent in terms of a hydrogenationreaction speed.

Patent Document 6: Japanese Patent No. 3490871

Further, other than the above-mentioned patent documents, as hydrogenabsorbing alloys for providing hydrogen absorbing alloy electrodes withhigh capacities have been proposed many kinds of rare earthelement-Mg—Ni based hydrogen absorbing alloys and hydrogen absorbingalloys containing the respective elements of rare earth element-Mg—Nibased alloys as main constituent elements and additionally elements suchas Cu, Co, Mn, and Al as other constituent elements (e.g. PatentDocuments 7 to 10).

Patent Document 7: JP-A No. 2000-80429

Patent Document 8: JP-A No. 2004-115870

Patent Document 9: JP-A No. 2000-265229

Patent Document 10: JP-A No. 2000-21439

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, although these rare earth-Mg—Ni based alloys known asconventional techniques are excellent in easiness of attaining excellentdischarge capacities as compared with AB₅ type rare earth based alloysin the case of being used for hydrogen absorbing electrodes, thesealloys have a problem that the alloys are easy to decrease in thedischarge capacity and discharge speed in the case charging anddischarging are repeated and thus inferior in the cycle characteristicsand rate characteristics.

Further, it is required for the AB₅ type rare earth based alloys to usecostly Co as an indispensable component to increase the cyclecharacteristics and in this case, it results in a problem that theproduction cost is high.

In view of the above problems of the conventional techniques, it is oneobject of the present invention to provide a hydrogen absorbing alloyhaving a high capacity and excellent in the cycle characteristics.Further, another object of the present invention is to provide ahydrogen absorbing alloy having a high capacity and excellent in thecycle characteristics economically.

Means for Solving the Problems

The present inventors have made various investigations to solve theabove-mentioned problems and have found that a hydrogen absorbing alloycontaining a certain specific crystal structure has a high capacity andis excellent in durability to charging and discharging cycles and havefinally accomplished the invention.

That is, the present invention provides a hydrogen absorbing alloycontaining a phase of a Pr₅Co₁₉ type crystal structure having acomposition defined by a general formula A_((4−w))B_((1+w))C₁₉ (Adenotes one or more element(s) selected from rare earth elementsincluding Y (yttrium); B denotes an Mg element; C denotes one or moreelement(s) selected from a group consisting of Ni, Co, Mn, and Al; and wdenotes a numeral in a range from −0.1 to 0.8) and having a compositionas a whole defined by a general formula R1_(x)R2_(y)R3_(z) (15.8≦x≦17.8,3.4≦y≦5.0, 78.8≦z≦79.6, and x+y+z=100; R1 denotes one or more element(s)selected from rare earth elements including Y (yttrium); R2 denotes anMg element, R3 denotes one or more element(s) selected from a groupconsisting of Ni, Co, Mn, and Al; the numeral of Mn+Al in the z is 0.5or higher; and the numeral of Al in the z is 4.1 or lower).

The hydrogen absorbing alloy of the present invention contains the phaseof the Pr₅Co₁₉ crystal structure which is completely different fromthose disclosed in the above-mentioned patent documents, so that thealloy has a high capacity and is excellent in durability to charging anddischarging cycles.

Herein, x, y, and z denote the ratios based on numbers of elements butnot weight %.

Further, the above-mentioned Pr₅Co₁₉ type crystal structure is ahexagonal crystal system belonging to a space group of P6₃/mmc and has aratio of c-axis length/a-axis length of the lattice constant in a rangeof 6.2 to 6.6.

Further, the present invention also provides the hydrogen absorbingalloy wherein in the general formula R1_(x)R2_(y)R3_(z), x, y, and zsatisfy 16.3≦x≦17.6, 3.6≦y≦4.7, and 78.8≦z≦79.1; the numeral of Mn+Al inthe z is 1.6 or higher; and the numeral of Al in the z is 1.9 or lower.

If the configuration is as described above, the formation ratio of thephase of the Pr₅Co₁₉ crystal structure can be increased and the hydrogenabsorbing alloy can have a high capacity and is more excellent in thedurability to charging and discharging cycles.

Further, the hydrogen absorbing alloy according to the present inventionis preferable to contain 8 weight % or more of the phase of the Pr₅Co₁₉crystal structure.

If the configuration is as described above, the alloy has a furtherhigher capacity and is more excellent in the durability to charging anddischarging cycles.

The present invention also provides a hydrogen absorbing alloyproduction method for producing the above-mentioned hydrogen absorbingalloy by cooling a melted alloy at a cooling speed of 1000 K/second ormore and further annealing the obtained alloy at a temperature in arange from 860 to 1020° C. in an inert gas atmosphere under apressurized state.

According to the above-mentioned production method, the formation ratioof the phase of the Pr₅Co₁₉ crystal structure, which is a metastablephase, can be increased and it is made possible to obtain a hydrogenabsorbing alloy with a high capacity.

Herein, the melted alloy means a substance obtained by weighingprescribed amounts of raw material ingots (materials) based on thecomposition of the intended hydrogen absorbing alloy and heating andmelting these materials.

Further, the present inventors of have found that during production ofthe above-mentioned hydrogen absorbing alloy containing the phase of thePr₅Co₁₉ type crystal structure, the production of the phase of thePr₅Co₁₉ type crystal structure can be promoted by melting annealing thealloy in the state that a prescribed ratio or more of Cu is added andaccordingly have accomplished the present invention.

That is, the present invention provides a hydrogen absorbing alloycontaining 15 weight % or higher of a phase of a Pr₅Co₁₉ type crystalstructure and obtained by melting and annealing in the state that thecontent of a Cu element is controlled to be 1 to 8 mol %.

Since being produced by melting and annealing in the state that thecontent of a Cu element, which is an indispensable component of thealloy, is controlled to be 1 to 8 mol %, the hydrogen absorbing alloy ofthe present invention contains the phase of the Pr₅Co₁₉ type crystalstructure at a ratio of 15 weight % or higher and accordingly isprovided with a high hydrogen storage capacity and the excellent cyclecharacteristics. Further, as compared with Co, Cu is relativelyeconomical and therefore, the hydrogen absorbing alloy having theabove-mentioned excellent properties can be provided at a relatively lowcost.

In the present invention, the composition of the alloy as a whole ispreferably defined by a general formula R1_(a)R2_(b)R3_(c)Cu_(d) (R1denotes one or more element(s) selected from rare earth elementsincluding Y (yttrium); R2 denotes one or more element(s) selected from agroup consisting of Mg, Ca, Sr, and Ba; R3 denotes one or moreelement(s) selected from a group consisting of Ni, Co, Mn, Al, Fe, Cr,Zn, Si, Sn, V, Nb, Ta, Ti, Zr, and Hf, and a, b, c, and d denotenumerals satisfying 15≦a≦19, 2≦b≦7, 70≦c≦80, 1≦d≦7, and a+b+c+d=100,respectively).

With respect to the hydrogen absorbing alloy of the present invention,it is preferable that the R2 is Mg and the R3 is one or more element(s)selected from a group consisting of Co, Mn, Al, and Ni.

Further, the present invention provides a hydrogen absorbing alloyproduction method for producing the above-mentioned hydrogen absorbingalloy by cooling a melted alloy at a cooling speed of 1000 K/second ormore and further annealing the obtained alloy at a temperature in arange from 860 to 980° C. in an inert gas atmosphere in a pressurizedstate. The temperature range of annealing is preferably from 920 to 970°C.

The hydrogen absorbing alloy of the present invention is preferable tohave a primary grain size of the alloy of 10 to 100 nm.

Generally, if a hydrogen absorbing alloy is finely powdered (to have anextremely small particle size), the crystal structure tends to be easilybroken up and accordingly the durability is lowered, however if havingsuch a configuration, pulverization of the alloy at the time ofabsorbing and releasing hydrogen can be suppressed and thus thedurability is further improved.

The hydrogen absorbing alloy electrode of the present invention uses theabove-mentioned hydrogen absorbing alloy as a hydrogen storage mediumand the secondary battery of the present invention employs the hydrogenabsorbing alloy electrode as a negative electrode.

EFFECTS OF THE INVENTION

As described above, the hydrogen absorbing alloy of the presentinvention has a high hydrogen storage capacity in the case the alloy isused for a hydrogen absorbing electrode and excellent durability even inthe case charging and discharging are repeated.

The secondary battery of the present invention employs, as a negativeelectrode, the hydrogen absorbing alloy electrode using the hydrogenabsorbing alloy having the above-mentioned properties as a hydrogenstorage medium and accordingly is provided with a high capacity and anexcellent cycle characteristic as compared with those using aconventional AB₅ type rare earth based alloy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: A drawing showing a structure model of a Pr₅Co₁₉ type crystalstructure.

FIG. 2: A drawing showing a transmission electron microscopic photograph(TEM image) of secondary particles.

FIG. 3: A drawing showing a relation between the heat treatmenttemperature and the existence ratio of the Pr₅Co₁₉ type crystalstructure phase.

FIG. 4: A drawing showing a relation between the heat treatmenttemperature and the capacity retention ratio.

FIG. 5: A drawing showing a relation between the heat treatmenttemperature and the existence ratio of the Pr₅Co₁₉ type crystalstructure phase.

FIG. 6: A drawing showing a relation between the heat treatmenttemperature and the capacity retention ratio.

FIG. 7: A drawing showing a relation between the heat treatmenttemperature and the existence ratio of the Pr₅Co₁₉ type crystalstructure phase.

FIG. 8: A drawing showing a relation between the heat treatmenttemperature and the capacity retention ratio.

FIG. 9: A drawing showing a relation between the heat treatmenttemperature and the existence ratio of the Pr₅Co₁₉ type crystalstructure phase.

FIG. 10: A drawing showing a relation between the heat treatmenttemperature and the capacity retention ratio.

BEST MODE FOR CARRYING OUT THE INVENTION

A first embodiment of the hydrogen absorbing alloy of the presentinvention contains a phase of a Pr₅Co₁₉ type crystal structure having acomposition defined by a general formula A_((4−w))B_((1+w))C₁₉ (Adenotes one or more element(s) selected from rare earth elementsincluding Y (yttrium); B denotes an Mg element; C denotes one or moreelement(s) selected from a group consisting of Ni, Co, Mn, and Al; and wdenotes a numeral in a range from −0.1 to 0.8).

Since the alloy contains the phase of a Pr₅Co₁₉ type crystal structurehaving a composition defined by the above-mentioned general formulaA_((4−w))B_((1+w))C₁₉, the resulting hydrogen absorbing alloy has a highcapacity and excellent durability to charging and discharging cycles.That is, the Pr₅Co₁₉ type crystal structure defined by the generalformula A_((4−w))B_((1+w))C₁₉, is a layered structure composed of threeAB₅ units of a CaCu₅ type crystal structure and one A_((1−w))B_((1+w))C₄unit of a Laves structure and since the element A (rare earth element)with a larger atom radium and the element B (Mg) with a smaller atomradium exist at a ratio of w=−0.1 to 0.8 in the A_((1−w))B_((1+w))C₄unit, the strains among the units are lowered to give a lattice volumesuitable for reversible hydrogen storage and release. If w is lower than−0.1, the ratio of the rare earth elements is increased, strains amongthe units are increased and the lattice volume is also increased, andaccordingly hydrides are to exist in a stable state to result indifficulty in the release of stored hydrogen. If w is higher than 0.8,the ratio of Mg is increased and the lattice constant is lowered tosupposedly result in difficulty in storing hydrogen.

The existence and the amount (weight %) of the phase of the Pr₅Co₁₉ typecrystal structure can be evaluated by carrying out x-ray diffraction of,for instance, a milled hydrogen absorbing alloy powder and analyzing theobtained x-ray diffraction pattern by a Rietveld method. Morespecifically, the existence and the amount can be measured by the methoddescribed in Examples.

FIG. 1 shows a structure model of the phase of the Pr₅Co₁₉ crystalstructure. According to the structure analysis by x-ray diffraction anda Rietveld method, it is understood that the above-mentioned phase ofthe Pr₅Co₁₉ crystal structure has the structure model shown in FIG. 1.

The specific crystal structure of the above-mentioned phase of thePr₅Co₁₉ type crystal structure is shown in the following.

The crystal system belongs to a hexagonal system and the space groupbelongs to P6₃/mmc.

As lattice parameters, the a-axis length is in a range from 4.980 to5.080 Å and the c-axis length is in a range from 30.88 to 33.53 Å.

The ratio of the c-axis length/a-axis length of the lattice constant is6.20 to 6.60 and V (volume) is 663.1 to 749.3 Å³.

The hydrogen absorbing alloy of the first embodiment of the presentinvention has a composition of the whole alloy defined by a generalformula R1_(x)R2_(y)R3_(z) (in the formula, 15.8≦x≦17.8, 3.4≦y≦5.0,78.8≦z≦79.6, and x+y+z=100; R1 denotes one or more element(s) selectedfrom rare earth elements including Y (yttrium); R2 denotes an Mgelement, R3 denotes one or more element(s) selected from a groupconsisting of Ni, Co, Mn, and Al; the numeral of Mn+Al in the z is 0.5or higher; and the numeral of Al in the z is 4.1 or lower). Further, itis preferable that x, y, and z in the general formula R1_(x)R2_(y)R3_(z)satisfy 16.3≦x≦17.6, 3.6≦y≦4.7, and 78.8≦z≦79.1; the numeral of Mn+Al inthe z is 1.6 or higher; and the numeral of Al in the z is 1.9 or lower.

The hydrogen absorbing alloy of the first embodiment of the presentinvention contains, as R1, one or more element(s) selected from rareearth elements including Y (yttrium), that is, a group consisting of La,Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, and Y and interms of the hydrogen dissociation equilibrium pressure, particularlyLa, Ce, Pr, and Nd are preferable.

Further, a Misch metal (Mm), a mixture of rare earth elements, iseconomical and therefore preferably used.

The hydrogen absorbing alloy of the first embodiment of the presentinvention contains, as R2, an Mg element. Use of an Mg element improvesthe hydrogen storage capacity and the durability.

The hydrogen absorbing alloy of the first embodiment of the presentinvention contains, as R3, at least one of Mn and Al and the numeral ofMn+Al (that is, the composition ratio of Mn+Al in the alloy as a whole)in the z is 0.5 or higher and preferably 1.6 or higher.

If the composition ratio of Mn+Al is 0.5 or higher, the productionamount of the phase of the Pr₅Co₁₉ crystal structure can be increased.

The composition ratio of Mn is preferably 0.2 to 3.7 and more preferably1.0 to 3.7.

If the ratio of Mn is within the above-mentioned range, it is madepossible to improve the capacity while keeping the phase of the Pr₅Co₁₉crystal structure.

Further, R3 is preferable to include Ni. Use of Ni as an indispensableelement makes the property of absorbing and releasing hydrogen better.

In the above-mentioned alloy composition, as described above, the ratioof Al is 4.1 or lower and preferably 1.9 or lower.

If the composition is within the above-mentioned range, the productionamount of the phase of the Pr₅Co₁₉ crystal structure can be increased.If the composition ratio of Al exceeds 4.1, the production amount of thephase of the Pr₅Co₁₉ crystal structure is considerably decreased and thehydrogen storage capacity and durability are deteriorated.

Specific compositions of the hydrogen absorbing alloy of the firstembodiment of the present invention may include, for instance,La_(16.9)Mg_(4.1)Ni_(69.2)Co_(6.0)Mn_(1.9)Al_(1.9),La_(16.9)Mg_(4.1)Ni_(71.1)Co_(5.8)Mn_(1.0)Al_(1.1),La_(17.0)Mg_(4.2)Ni_(77.0)Mn_(1.8),La_(17.2)Mg_(4.0)Ni_(73.3)Cu_(3.9)Mn_(1.6),La_(12.8)Pr_(4.0)Mg_(3.6)Ni_(72.1)Co_(4.4)Mn_(3.1),La_(13.4)Ce_(4.2)Mg_(3.6)Ni_(71.1)Co_(4.0)Mn_(3.7),La_(13.9)Ce_(2.1)Nd_(0.8)Mg_(4.1)Ni_(72.2)Co_(4.0)Mn_(2.9),La_(14.1)Ce_(2.0)Nd_(0.9)Mg_(4.1)Ni_(76.3)Mn_(2.6),La_(17.6)Mg_(3.6)Ni_(76.4)Mn_(1.4)Al_(1.0),La_(17.8)Mg_(3.4)Ni_(68.2)Co_(6.4)Mn_(2.1)Al_(2.1),La_(16.3)Mg_(4.7)Ni_(69.9)Cu_(6.0)Mn_(2.0)Al_(1.1),La_(15.8)Mg_(5.0)Ni_(71.1)Co_(4.1)Mn_(2.0)Al_(2.0),La_(17.0)Mg_(4.1)Ni_(73.9)Mn_(0.9)Al_(4.1), andLa_(17.0)Mg_(4.1)Ni_(73.7)Cu_(4.7)Mn_(0.2)Al_(0.3).

The hydrogen absorbing alloy of the first embodiment of the presentinvention contains the phase of the Pr₅Co₁₉ type crystal structure andgenerally the phase of the Pr₅Co₁₉ type crystal structure exists in anamount of 8 weight % or more, preferably 65 weight % or more, and morepreferably 79 weight % or more in the alloy. The upper limit is notparticularly limited, however it is generally about 95 weight %.

The higher the existence ratio of the phase of the Pr₅Co₁₉ type crystalstructure becomes, the higher the capacity becomes.

The phase of the Pr₅Co₁₉ type crystal structure has, as shown in thecrystal structure model in FIG. 1, the stacking structure of the AB₂units and the AB₅ units and the ratio of the AB₅ units, which havehigher durability, is higher and it is supposed that Mn or Al added inthe alloy moderates the strains caused between neighboring AB₂ units andAB₅ units and accordingly the stability of the crystal structure isincreased to improve the durability of the alloy.

Further, the strains in other produced phases and grain boundaries aremoderated to suppress pulverization. Since pulverization is suppressed,the contact surface area of an alkaline electrolyte and the hydrogenabsorbing alloy is lessened and accordingly, corrosion of the hydrogenabsorbing alloy is suppressed, the cycle life is improved, and excellentdurability is provided.

Further, it is also supposed that, as compared with the conventionalCeNi₃ type and PuNi₃ type, since the Mg content contained in the crystalis low, the alkali resistance is improved.

The existence ratio (weight %) of the phase of the Pr₅Co₁₉ type crystalstructure in the hydrogen absorbing alloy of the present invention canbe measured by the method described in Examples.

As other produced phases, phases of crystal structures such as a CeNi₃type, a PuNi₃ type, a Ce₂Ni₇ type, Ce₅Co₁₉ type, and a CaCu₅ type can beexemplified.

The hydrogen absorbing alloy of one embodiment of the present inventionpreferably has a primary grain size of the alloy in a range of 10 to 100nm.

Control of the primary grain size within the range of 10 to 100 nmmoderates volume expansion caused along with hydrogen absorption andmakes occurrence of pulverization difficult. Further, phasetransformation due to rearrangement of atoms is easily caused duringheat treatment to produce the phase of the Pr₅Co₁₉ crystal structureeasily.

In the case the primary grain size exceeds 100 nm, the cyclecharacteristic is deteriorated due to pulverization and in the case itis smaller than 10 nm, deterioration due to oxidation tends to be causedeasily.

The primary grain size can be measured by transmission electronmicroscopic observation described in Examples.

The hydrogen absorbing alloy according to the first embodiment of thepresent invention is preferable to have an average particle size ofsecondary grains in a range of 20 to 60 μm.

If the average particle size of the secondary grains is within theabove-mentioned range, a good high rate discharge characteristics can beobtained and also corrosion by a strongly alkaline electrolyte can besuppressed and the durability is further improved.

The secondary grains mean grains of polycrystalline bodies formed bybonding a plurality of primary grains.

FIG. 2 shows a transmission electron microscopic photograph (TEM image)of the secondary grains.

The hydrogen absorbing alloy of one embodiment of the present inventioncan be obtained by mixing raw material ingots (materials) in properamounts to give a prescribed alloy composition, melting the ingots, andannealing the coarse product, which is obtained by cooling andsolidifying the melt, in a temperature range of 860 to 1020° C.

First, prescribed amounts of raw material ingots (materials) are weighedin accordance with the composition of the intended hydrogen absorbingalloy and the raw material ingots are put in a crucible and heated to1200 to 1600° C. using a high frequency melting furnace in an inert gasatmosphere or vacuum to melt the materials.

Thereafter, the melted alloy obtained by melting the above-mentionedmaterials is cooled and at this time, the cooling speed for cooling themelted alloy is adjusted to be 1000 K/second or more.

Quenching of the melted alloy at 1000 k/second or more in such a mannermakes it possible to produce an alloy phase (the phase of the Pr₅Co₁₉crystal structure) of the present invention, which is a metastablephase, at high efficiency.

A cooling method for solidification by quenching may be a melt spinningmethod capable of quenching at a cooling speed of 100,000 K/second ormore.

Another usable cooling method may be, for example, a mold method(capable of cooling at a cooling speed of 1,000 K/second) or a gasatomizing method (capable of cooling at a cooling speed of 10,000K/second).

Next, after quenching solidification of the alloy, to improve theproduction ratio of the intended alloy phase, heat treatment is carriedout using an electric furnace in an inert gas atmosphere in apressurized state. The heat treatment (annealing) to be carried out inthe electric furnace is preferably performed at a temperature in thefurnace in a range of 860 to 1020° C. for 3 to 50 hours.

The lower temperature limit and the upper temperature limit of theabove-mentioned heat treatment are defined as 860° C. and 1020° C.,respectively, to improve the production ratio of the phase of thePr₅Co₁₉ type crystal structure.

The heat treatment is carried out for 3 hours at minimum, so that theintended phase of the Pr₅Co₁₉ crystal structure can be produced at ahigh ratio. Heat treatment for a duration exceeding 50 hours is notpreferable since a stable phase such as a phase of a LaNi₅ crystalstructure appears.

The heat treatment conditions are more preferably at 900 to 980° C. for5 to 10 hours.

The reason that the heat treatment atmosphere is controlled to be aninert gas (e.g. argon or helium) atmosphere in a pressurized state isbecause oxidation of the material can be prevented and at the same timeevaporation of magnesium can be prevented during the heat treatment.

Particularly, a helium gas atmosphere is preferable since the effect ofpreventing magnesium evaporation is significant.

At the time of carrying out the heat treatment under an inert gas, thepressure range is 0.1 MPa or higher and preferably in a range of 0.2 to0.5 MPa (gauge pressure).

If the heat treatment is carried out in the above-mentioned pressurerange, it is made possible to obtain a hydrogen absorbing alloy moreexcellent in durability.

In this connection, even if the heat treatment is carried out in thepressure range, the heat treatment temperature is preferably 900 to 980°C.

It is made possible to obtain a hydrogen absorbing alloy containing alarge quantity of the phase of the Pr₅Co₁₉ crystal structure by carryingout the heat treatment at the above-mentioned temperature and durationin an inert gas atmosphere in a pressurized state.

Next, the hydrogen absorbing alloy of a second embodiment of the presentinvention will be described.

The hydrogen absorbing alloy according to the second embodiment of thepresent invention is obtained by melting and annealing alloy rawmaterials to have a content of a Cu element in a range of 1 to 8 mol %and forming the phase of the Pr₅Co₁₉ type crystal structure at a ratioof 15 weight % or higher. The phase of the Pr₅Co₁₉ type crystalstructure is the same as described in the first embodiment.

The phase of the Pr₅Co₁₉ type crystal structure is intrinsically ametastable phase, however it is stabilized by addition of a Cu elementand the production ratio is remarkably increased. Although the cause ofremarkable increase of the production ratio is unclear, it is supposeddue to that specific element sites of Ni or the like sandwiched betweenrare earth element sites or Group IIA element (Mg, Ca, Sr, or Ba) sitesare selectively replaced with Cu.

Further, it is also supposed that the alloy corrosion is suppressed andthe cycle characteristic is improved since the phase of the Pr₅Co₁₉ typecrystal structure is stable to an alkaline electrolyte and the primarygrains of the crystal phases of the Pr₅Co₁₉ type crystal structure existin grain boundaries of primary grains of other crystal phases of theCe₂Ni₇ type crystal phase or the like and the crystal phases of theCe₂Ni₇ type crystal phase or the like decrease chances of the directcontact of the primary grains with an electrolyte.

Further, the element such as Ni in the crystal is replaced with a Cuelement, so that the hydrogen dissociation equilibrium pressure can belowered and at the same time the conductivity of the alloy can beimproved to make the alloy usable for high current electric discharge.

In the hydrogen absorbing alloy according to the second embodiment ofthe present invention, the content of a Cu element is 1 to 8 mol % andmore preferably 2 to 7 mol %. If the content of a Cu element is 2 to 7mol %, the production amount of the above-mentioned phase of the Pr₅Co₁₉type crystal structure is further increased and the above-mentionedeffects of the present invention can be caused particularlysignificantly.

If the content of Cu exceeds 8%, the production ratio of theabove-mentioned phase of the Pr₅Co₁₉ type crystal structure may belowered and the cycle characteristics may be deteriorated in some cases.It is supposedly attributed to that if the content of Cu exceeds 8%, theproduction ratio of other crystal phases (e.g. Ce₂Ni₇ type crystalphase, Ce₅Ni₁₉ type crystal phase, and AuBe₅ type crystal phase)inferior in corrosion resistance to an alkaline electrolyte isincreased.

The hydrogen absorbing alloy according to the second embodiment of thepresent invention contains 15 weight % or more of the phase of thePr₅Co₁₉ type crystal structure and the ratio of the phase is morepreferably 25 weight % or more, furthermore preferably 40 weight %, andespecially preferably in a range of 50 to 85 weight %.

As the existence ratio of the Pr₅Co₁₉ type crystal structure is higher,the grain boundaries with other produced phases are lessened more andaccordingly the strains during expansion and shrinkage can be moderatedand pulverization can be suppressed. Owing to the suppression of thepulverization, the contact surface area of the hydrogen absorbing alloywith the alkaline electrolyte can be lessened and the corrosion of thehydrogen absorbing alloy can be suppressed and accordingly, the cyclelife can be improved.

The hydrogen absorbing alloy according to the second embodiment of thepresent invention is preferable to have a chemical composition definedby a general formula R1_(a)R2_(b)R3_(c)Cu_(d) (in the formula, R1denotes one or more element(s) selected from rare earth elementsincluding Y (yttrium); R2 denotes one or more element(s) selected from agroup consisting of Mg, Ca, Sr, and Ba; R3 denotes one or moreelement(s) selected from a group consisting of Ni, Co, Mn, Al, Fe, Cr,Zn, Si, Sn, V, Nb, Ta, Ti, Zr, and Hf; and a, b, c, and d denotenumerals satisfying 15≦a≦19, 2≦b≦7, 70≦c≦80, 1≦d≦7, and a+b+c+d=100,respectively).

As the rare earth elements for the R1, one or more element(s) selectedfrom a group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er,Tm, Yb, Lu, Sc, and Y are used and in terms of the hydrogen dissociationequilibrium pressure, particularly one or more element(s) selected froma group consisting of La, Ce, Pr, and Nd are preferable. Especially, aMisch metal (Mm), a mixture of rare earth elements, is economical andtherefore preferably used.

As the R2, one or more element(s) selected from a group consisting ofMg, Ca, Sr, and Ba are used, and in terms of the hydrogen storagecapacity and the corrosion resistance, Mg is particularly preferable.

As the R3, one or more element(s) selected from a group consisting ofNi, Co, Mn, Al, Fe, Cr. Zn, Si, Sn, V, Nb, Ta, Ti, Zr, and Hf are used,and particularly one or more element(s) selected from a group consistingof Co, Mn, Al and Ni are preferably used. Use of Ni and Co provides thehydrogen absorbing alloy with excellent alkali resistance and electrodecatalytic activity. Further, use of Mn and Al makes it possible toadjust the hydrogen dissociation equilibrium pressure and particularly,it is preferable to add at least one of Mn and Al at a ratio in a rangeof 0.3 to 0.6 mol % (in the case of adding both, the total amount).

Examples of the specific chemical composition of the hydrogen absorbingalloy of the present invention may include La_(a)Mg_(b)(NiMn)_(c)Cu_(d),La_(a)Mg_(b)(NiAl)_(c)Cu_(d), and the like.

Herein, a, b, c, and d are the numerals same as those in theabove-mentioned general formula.

The hydrogen absorbing alloy of the second embodiment of the presentinvention is preferable to have a particle size of the primary grains ina range from 10 to 100 nm.

Control of the particle size of the primary grains in the range of 10 to100 nm causes the following effects: (1) since the phase of the Pr₅Co₁₉type crystal structure is evenly dispersed in grain boundaries of othercrystal phases such as a phase of the Ce₂Ni₇ type crystal structure, thevolume change along with the hydrogen absorption can efficiently bemoderated and pulverization can be prevented and accordingly thecorrosion resistance is remarkably improved and (2) the phase changealong with heat treatment is easily caused and the alloy phase of thepresent invention can be obtained at high efficiency. If the crystalparticle size is smaller than 10 nm, oxidation tends to be caused easilyand if it exceeds 100 nm, pulverization tends to occur easily.

To control the particle size of the primary grains within theabove-mentioned range, a method involving quenching melted materials ata cooling speed of 100000K/second or more by a melt spinning method andannealing the materials under conditions described later can preferablybe employed.

In this connection, the phrase that the particle size of the primarygrains is in a range of 10 to 100 nm means that almost all of theprimary grains are contained in a range of 10 nm at minimum and 100 nmat maximum. More specifically, it means that, in the case the particlesize is measured for arbitrary 100 grains in an electron microscopicphotograph, the ratio of the grains having the particle size in a rangeof 10 to 100 nm is 80% or more on the basis of the surface area.Further, primary grains mean grains having the single crystal structurecomposed of a single crystallite (also called as crystal grains). Theparticle size of each primary grain can be measured by using atransmission electron microscope. Specifically, a transmission electronmicroscope (Hitachi H9000) is used for measuring the longest length ofthe long side and the shortest length of the short side of each crystalgrain and the particle size of the primary grain is calculated accordingto the following equation.

Particle size of primary grain=(long side+short side)/2

The hydrogen absorbing alloy of the second embodiment of the presentinvention can be obtained by mixing alloy materials in proper amounts togive the above-mentioned chemical composition defined by the generalformula, heating and melting the materials, quenching and solidifyingthe melt at a cooling speed of 1000 K/second or more, and thereafterannealing the obtained coarse product in a temperature range of 860 to980° C., preferably in a temperature range of 920 to 970° C. under aninert gas atmosphere in a pressurized state.

More specifically, first, prescribed amounts of raw material ingots(alloy materials) are weighed in accordance with the composition of anintended hydrogen absorbing alloy and the raw material ingots are put ina crucible and heated to 1200 to 1600° C. using a high frequency meltingfurnace in an inert gas atmosphere or vacuum to melt the materials.Thereafter, the melted materials are cooled for solidification.

The cooling speed at the time of cooling the melted materials ispreferably 1000 K/second or more (also called as quenching). Although astable phase such as a CaCu₅ type crystal structure tends to be formedeasily in the case the cooling speed is lower than 1000 K/second, it ismade easy to produce the Pr₅Co₁₉ phase, which is a metastable phase, byquenching at 1000 K/second or more. In terms of this point, the coolingmethod used is preferably a melt spinning method capable of cooling at acooling speed of 100,000 K/second or higher and a gas atomizing methodcapable of cooling at a cooling speed of about 10,000 K/second.

Next, after solidification of the alloy, heat treatment (annealing) iscarried out using an electric furnace in an inert gas atmosphere.Specifically, recrystallization annealing is carried out at a heattreatment temperature in a range of 860 to 980° C. for 3 to 50 hoursunder an inert gas atmosphere in a pressurized state of 0.2 to 1.0 MPa(gauge pressure).

After the cooling solidification, the alloy contains a mixed phase ofthe Pr₅Co₁₉ type crystal structure phase, which is a metastable phase,and a stable phase, however, the above-mentioned heat treatment canremarkably increase the existence ratio of the Pr₅Co₁₉ type crystalstructure phase.

Conventionally, homogenization treatment has been carried out by heattreatment at 1000 to 1100° C. in vacuum; however, it is probable thatthe heat treatment under such a condition easily volatilizes Mg and maycause crystal structure change. In the present invention, thetemperature condition at the time of heat treatment is set preferably to860 to 980° C. and the pressure condition at the time of the heattreatment is set preferably to 0.2 to 1.0 MPa (gauge pressure). Further,the atmosphere used at the time of heat treatment is controlledpreferably to be an inert atmosphere of argon or helium and particularlypreferably a helium gas is used. If the heat treatment is carried outunder such conditions, the Pr₅Co₁₉ phase can be obtained at highefficiency.

The heat treatment atmosphere is set to be an inert gas atmosphere (e.g.argon or helium) because oxidation of materials is prevented during theheat treatment. Further, the heat treatment temperature is 860 to 980°C. and particularly preferably 920 to 970° C. If the heat treatmenttemperature is within the range of 920 to 970° C., the ratio of thePr₅Co₁₉ type crystal structure phase is remarkably increased and thePr₅Co₁₉ type crystal structure phase becomes a main phase (the phasewhich occupies the largest part in the alloy).

The hydrogen absorbing alloy electrode of the second embodiment of thepresent invention is provided with the above-mentioned hydrogenabsorbing alloy as a hydrogen storage medium. In the case of using thehydrogen absorbing alloy according to the second embodiment of thepresent invention as a hydrogen storage medium for an electrode, thehydrogen absorbing alloy is preferably used after being ground.

Grinding of the hydrogen absorbing alloy at the time of producing anelectrode may be carried out either before or after the heat treatment,however since the surface area is increased by the grinding, it isdesirable to carry out the grinding after the heat treatment in terms ofprevention of oxidation of the alloy surface. The grinding is preferablycarried out in an inert atmosphere for preventing oxidation of the alloysurface.

For the grinding, for example, mechanical pulverization or hydrogenationpulverization may be employed.

The secondary battery according to the present invention uses thehydrogen absorbing alloy electrode as a negative electrode and isassembled as a nickel-hydrogen storage battery, for example. Thehydrogen absorbing alloy, that is, the hydrogen absorbing alloyelectrode of the present invention has corrosion resistance to astrongly alkaline aqueous solution to be used as an electrolyte for anickel-hydrogen storage battery or the like, so that the hydrogenabsorbing alloy electrode is excellent in the cycle characteristics ofrepeating absorption and release of hydrogen.

A nickel electrode (sintered type or non-sintered type) may be used as apositive electrode for the secondary battery.

EXAMPLES

Hereinafter, the present invention will be described more specificallywith reference to Examples and Comparative Examples, however it is notintended that the present invention be limited to the followingExamples. The various kinds of characteristics were measured by thefollowing methods.

(Measurement Method of Crystal Grain (Primary Grain) Size)

The crystal grain (primary grain) size was measured using a transmissionelectron microscope (Hitachi H9000).

The crystal grain size was measured by measuring the longest length ofthe long side and the shortest length of the short side of each crystalgrain for arbitrary 100 pieces using a transmission electron microscopeand calculated according to the following equation.

Crystal grain size=(long side+short side)/2

(Measurement Method of Average Particle Size)

The average particle size and particle size distribution of the hydrogenabsorbing alloys were measured by a laser diffraction/scattering methodusing a particle size analyzer (product number: MT3000, manufactured byMicroTrack Co., Ltd.).

In this connection, the average particle size means a progressiveaverage diameter D50, that is, the particle size at 50% point of thecumulative curve formed by setting the entire volume of the powder to be100%.

(Measurement of Crystal Structure and Measurement of Existence Ratio)

Each obtained hydrogen absorbing alloy was pulverized to obtain a powderwith an average particle size (D50) of 20 μm and using an x-raydiffraction apparatus (product number: M06XCE, manufactured by BrukerAXS) and in conditions of 40 kV and 100 mA (Cu tube), the measurementwas carried out for the powder. Analysis was carried out by a Rietveldmethod (analysis software: RIETAN 2000) as structure analysis.

(Measurement of Discharge Capacity) [Production of Electrode]

After 3 parts by weight of a nickel powder (#210, manufactured by INCO)was added and mixed to 100 parts by weight of an alloy powder, themixture was further mixed with an aqueous solution containing athickener (methyl cellulose) dissolved therein and also 1.5 parts byweight of a binder (styrene-butadiene rubber) to produce a paste, andthe paste was applied to both faces of a perforated steel plate with athickness of 45 μm (porosity: 60%) and dried and the resulting steelplate was pressed to a thickness of 0.36 mm to obtain a negativeelectrode. On the other hand, as a positive electrode, a sintered typenickel hydroxide electrode with an excess capacity was employed.

[Production of Opened Type Battery]

The negative electrode was sandwiched between the positive electrodeswith a separator interposed therebetween and fixed by acrylic plates ina manner that a pressure of 10 kgf/cm² could be applied to theseelectrodes to assemble an opened type cell.

As an electrolyte was employed a mixed solution containing 6.8 mol/L ofa KOH solution and 0.8 mol/L of an LiOH solution.

[Measurement of Discharge Capacity]

Each produced battery was put in a water bath at 20° C. and 10 cycles ofcharging and discharging were carried out in the following conditions:charging to 150% of the capacity at 0.1 ItA; discharging to the finalvoltage of −0.6 V (vs. Hg/HgO) at 0.2 ItA and when the capacity becamethe maximum, the discharge capacity was measured.

(Measurement of Charging and Discharging Cycle Characteristics)

Using each opened type battery produced in the above-mentioned

manner, in a water bath at 20° C. 10 cycles of charging and dischargingwere repeated in the following conditions: charging to 150% of thecapacity at 0.1 ItA; discharging to the final voltage of −0.6 V (vs.Hg/HgO) at 0.2 ItA.

At the 11th cycle, charging and discharging were carried out inconditions of charging to 150% at 0.1 ItA and discharging to the finalvoltage of −0.6 V (vs. Hg/HgO) at 1 ItA: and at the 12th cycle, chargingand discharging were carried out in conditions of charging to 150% at0.1 ItA and discharging to the final voltage of −0.6 V (vs. Hg/HgO) at 3ItA.

From the 13th cycle to 52nd cycle, charging and discharging were carriedout in conditions of charging to 75% at 1 ItA and discharging to thefinal voltage of −0.6 V (vs Hg/HgO) at 0.5 ItA.

From the 53rd cycle to 55th cycle, charging and discharging were carriedout in conditions of charging to 150% at 0.1 ItA and discharging to thefinal voltage of −0.6 V (vs Hg/HgO) at 0.2 ItA.

[Method for Measuring Cycle Deterioration Ratio]

The cycle deterioration ratio was measured according to the followingequation: Cycle deterioration ratio=(capacity at 53th cycle/capacity at10th cycle)×100.

(Method for Measuring Corrosion Resistance)

Each negative electrode was washed with water and dried after thecharging and discharging test and the saturated magnetization per massand the specific surface area were measured.

[Method for Measuring Saturated Magnetization per Mass]

The saturated magnetization per mass was measured using a vibratingsample magnetometer (VSM) (model name: BHV-10H, manufactured by RikenDenshi).

The saturated magnetization per mass of each hydrogen absorbing alloybefore the charging and discharging test was 0.2 Am²/kg or lower andalong with progress of corrosion, the value of the saturatedmagnetization per mass becomes higher.

(Measurement of Specific Surface Area)

The specific surface area was measured by a BET method (model name:direct-reading type full automatic surface area measurement apparatusMonosorb MS-19, manufactured by QUANTACHROME).

The specific surface area of the hydrogen absorbing alloy before thecharging and discharging test was 0.1 m²/g or lower and along withprogress of corrosion, the specific surface area becomes larger.

Example 1

In accordance with an intended composition, prescribed amounts of rawmaterial ingots were weighed and put in a crucible and heated to 1500°C. using a high frequency melting furnace in a helium gas atmosphere atreduced pressure to melt the materials. After the melting, the meltedalloy was solidified by a die casting method involving leaving the alloyin a die in the furnace.

Next, the obtained alloy was heat-treated at atmospheric pressure in ahelium gas atmosphere using an electric furnace. While the temperaturein the electric furnace is at 940° C., recrystallization annealing wascarried out for 7 hours, and successively, the alloy was spontaneouslycooled in the furnace to obtain a hydrogen absorbing alloy with acomposition of La_(16.9)Mg_(4.1)Ni_(71.1)Co_(5.8)Mn_(1.0)Al_(1.1).

Pulverization of the hydrogen absorbing alloy was carried outmechanically in a helium gas atmosphere to adjust the average particlesize to D50=60 μm. The particle size distribution width was as follows:D10=16 μm and D90=125 μm.

The crystal structure and the existence ratio (weight %) of the obtainedhydrogen absorbing alloy were measured. Further, using the obtainedhydrogen absorbing alloy, the discharge capacity at 10th cycle, thedischarge capacity at 12th cycle, the cycle deterioration ratio, thesaturated magnetization per mass after the cycle test, and the specificsurface area after the cycle test were measured.

Example 2

A hydrogen absorbing alloy with a composition ofLa_(16.9)Mg_(4.1)Ni_(71.1)Co_(5.8)Mn_(1.0)Al_(1.1) was obtained bycarrying out the same operation as that of Example 1, except thatprescribed amounts of raw material ingots were weighed in accordancewith the intended composition and recrystallization annealing wascarried out at an annealing temperature of 940° C. under 0.2 MPa for anannealing time of 7 hours.

The obtained hydrogen absorbing alloy was subjected to the samemeasurements as those carried out in Example 1.

Example 3

In accordance with an intended composition, prescribed amounts of rawmaterial ingots were weighed and put in a crucible and heated to 1500°C. using a high frequency melting furnace in a helium gas atmosphere atreduced pressure to melt the materials. After the melting, the meltedalloy was solidified by quenching at 100,000 K/second or more by a meltspinning method.

Next, the obtained alloy was recrystallization-annealed at a temperatureof 860° C. under 0.2 MPa in a helium gas atmosphere using an electricfurnace for 7 hours and successively, the alloy was spontaneously cooledin the furnace to obtain a hydrogen absorbing alloy with a compositionof La_(16.9)Mg_(4.1)Ni_(71.1)Co_(5.8)Mn_(1.0)Al_(1.1). The obtainedhydrogen absorbing alloy was subjected to the same measurements as thosecarried out in Example 1.

Example 4

A hydrogen absorbing alloy with a composition ofLa_(16.9)Mg_(4.1)Ni_(71.1)Cu_(5.8)Mn_(1.0)Al_(1.1) was obtained bycarrying out the same operation as that of Example 3, except thatprescribed amounts of raw material ingots were weighed in accordancewith the intended composition and recrystallization annealing wascarried out at an annealing temperature of 900° C. under 0.2 MPa for 7an annealing time of hours. The same measurements as those carried outin Example 1 were carried out for the obtained hydrogen absorbing alloy.

Example 5

A hydrogen absorbing alloy with a composition ofLa_(16.9)Mg_(4.1)Ni_(71.1)Co_(5.8)Mn_(1.0)Al_(1.1) was obtained bycarrying out the same operation as that of Example 3, except thatprescribed amounts of raw material ingots were weighed in accordancewith the intended composition and recrystallization annealing wascarried out at an annealing temperature of 940° C. under 0.2 MPa for anannealing time of 7 hours. The same measurements as those carried out inExample 1 were carried out for the obtained hydrogen absorbing alloy.

Example 6

A hydrogen absorbing alloy with a composition ofLa_(16.9)Mg_(4.1)Ni_(71.1)Co_(5.8)Mn_(1.0)Al_(1.1) was obtained bycarrying out the same operation as that of Example 3, except thatprescribed amounts of raw material ingots were weighed in accordancewith the intended composition and recrystallization annealing wascarried out at an annealing temperature of 980° C. under 0.2 MPa for anannealing time of 7 hours. The same measurements as those carried out inExample 1 were carried out for the obtained hydrogen absorbing alloy.

Example 7

A hydrogen absorbing alloy with a composition ofLa_(16.9)Mg_(4.1)Ni_(71.1)Co_(5.8)Mn_(1.0)Al_(1.1) was obtained bycarrying out the same operation as that of Example 3, except thatprescribed amounts of raw material ingots were weighed in accordancewith the intended composition and recrystallization annealing wascarried out at an annealing temperature of 1020° C. under 0.2 MPa for anannealing time of 7 hours.

The same measurements as those carried out in Example 1 were carried outfor the obtained hydrogen absorbing alloy.

Example 8

A hydrogen absorbing alloy with a composition ofLa_(16.9)Mg_(4.1)Ni_(71.1)Co_(5.8)Mn_(1.0)Al_(1.1) was obtained bycarrying out the same operation as that of Example 3, except thatprescribed amounts of raw material ingots were weighed in accordancewith the intended composition and recrystallization annealing wascarried out at an annealing temperature of 940° C. under 0.5 MPa for anannealing time of 7 hours.

The same measurements as those carried out in Example 1 were carried outfor the obtained hydrogen absorbing alloy.

Example 9

A hydrogen absorbing alloy with a composition ofLa_(16.9)Mg_(4.1)Ni_(71.1)Co_(5.8)Mn_(1.0)Al_(1.1) was obtained bycarrying out the same operation as that of Example 3, except thatprescribed amounts of raw material ingots were weighed in accordancewith the intended composition and recrystallization annealing wascarried out at an annealing temperature of 940° C. in a heliumatmosphere under atmospheric pressure for an annealing time of 7 hours.

The same measurements as those carried out in Example 1 were carried outfor the obtained hydrogen absorbing alloy.

Example 10

A hydrogen absorbing alloy with a composition ofLa_(16.9)Mg_(4.1)Ni_(69.2)Co_(6.0)Mn_(1.9)Al_(1.9) was obtained bycarrying out the same operation as that of Example 3, except thatprescribed amounts of raw material ingots were weighed in accordancewith the intended composition and recrystallization annealing wascarried out at an annealing temperature of 940° C. under 0.2 MPa for anannealing time of 7 hours.

The same measurements as those carried out in Example 1 were carried outfor the obtained hydrogen absorbing alloy.

Example 11

A hydrogen absorbing alloy with a composition ofLa_(17.0)Mg_(4.2)Ni_(77.0)Mn_(1.8) was obtained by carrying out the sameoperation as that of Example 3, except that prescribed amounts of rawmaterial ingots were weighed in accordance with the intended compositionand recrystallization annealing was carried out at an annealingtemperature of 940° C. under 0.2 MPa for 7 an annealing time of hours.

The same measurements as those carried out in Example 1 were carried outfor the obtained hydrogen absorbing alloy.

Example 12

A hydrogen absorbing alloy with a composition ofLa_(17.2)Mg_(4.0)Ni_(73.3)Co_(3.9)Mn_(1.6) was obtained by carrying outthe same operation as that of Example 3, except that prescribed amountsof raw material ingots were weighed in accordance with the intendedcomposition and recrystallization annealing was carried out at anannealing temperature of 940° C. under 0.2 MPa for an annealing time of7 hours.

The same measurements as those carried out in Example 1 were carried outfor the obtained hydrogen absorbing alloy.

Example 13

A hydrogen absorbing alloy with a composition ofLa_(12.8)Pr_(4.0)Mg₃₆Ni_(72.1)Co_(4.4)Mn_(3.1) was obtained by carryingout the same operation as that of Example 3, except that prescribedamounts of raw material ingots were weighed in accordance with theintended composition and recrystallization annealing was carried out atan annealing temperature of 940° C. under 0.2 MPa for an annealing timeof 7 hours.

The same measurements as those carried out in Example 1 were carried outfor the obtained hydrogen absorbing alloy.

Example 14

A hydrogen absorbing alloy with a composition ofLa_(13.4)Ce_(4.2)Mg_(3.6)Ni_(71.1)Co_(4.0)Mn_(3.7) was obtained bycarrying out the same operation as that of Example 3, except thatprescribed amounts of raw material ingots were weighed in accordancewith the intended composition and recrystallization annealing wascarried out at an annealing temperature of 940° C. under 0.2 MPa for anannealing time of 7 hours.

The same measurements as those carried out in Example 1 were carried outfor the obtained hydrogen absorbing alloy.

Example 15

A hydrogen absorbing alloy with a composition ofLa_(13.9)Ce_(2.1)Nd_(0.8)Mg_(4.1)Ni_(72.2)Co_(4.0)Mn_(2.9) was obtainedby carrying out the same operation as that of Example 3, except thatprescribed amounts of raw material ingots were weighed in accordancewith the intended composition and recrystallization annealing wascarried out at an annealing temperature of 940° C. under 0.2 MPa for anannealing time of 7 hours.

The same measurements as those carried out in Example 1 were carried outfor the obtained hydrogen absorbing alloy.

Example 16

A hydrogen absorbing alloy with a composition ofLa_(14.1)Ce_(2.0)Nd_(0.9)Mg_(4.1)Ni_(76.3)Mn_(2.6) was obtained bycarrying out the same operation as that of Example 3, except thatprescribed amounts of raw material ingots were weighed in accordancewith the intended composition and recrystallization annealing wascarried out at an annealing temperature of 940° C. under 0.2 MPa for anannealing time of 7 hours.

The same measurements as those carried out in Example 1 were carried outfor the obtained hydrogen absorbing alloy. The results are shown inTable 1.

Example 17

A hydrogen absorbing alloy with a composition ofLa_(17.6)Mg_(3.6)Ni_(76.4)Mn_(1.4)Al_(1.0) was obtained by carrying outthe same operation as that of Example 3, except that prescribed amountsof raw material ingots were weighed in accordance with the intendedcomposition and recrystallization annealing was carried out at anannealing temperature of 940° C. under 0.2 MPa for an annealing time of7 hours.

The same measurements as those carried out in Example 1 were carried outfor the obtained hydrogen absorbing alloy.

Example 18

A hydrogen absorbing alloy with a composition ofLa_(17.8)Mg_(3.4)Ni_(68.2)Co_(6.4)Mn_(2.1)Al_(2.1) was obtained bycarrying out the same operation as that of Example 3, except thatprescribed amounts of raw material ingots were weighed in accordancewith the intended composition and recrystallization annealing wascarried out at an annealing temperature of 940° C. under 0.2 MPa for anannealing time of 7 hours.

The same measurements as those carried out in Example 1 were carried outfor the obtained hydrogen absorbing alloy.

Example 19

A hydrogen absorbing alloy with a composition ofLa_(16.3)Mg_(4.7)Ni_(69.9)Co_(6.0)Mn_(2.0)Al_(1.1) was obtained bycarrying out the same operation as that of Example 3, except thatprescribed amounts of raw material ingots were weighed in accordancewith the intended composition and recrystallization annealing wascarried out at an annealing temperature of 940° C. under 0.2 MPa for anannealing time of 7 hours.

The same measurements as those carried out in Example 1 were carried outfor the obtained hydrogen absorbing alloy.

Example 20

A hydrogen absorbing alloy with a composition ofLa_(15.8)Mg_(5.0)Ni_(71.1)Co_(4.1)Mn_(2.0)Al_(2.0) was obtained bycarrying out the same operation as that of Example 3, except thatprescribed amounts of raw material ingots were weighed in accordancewith the intended composition and recrystallization annealing wascarried out at an annealing temperature of 940° C. under 0.2 MPa for anannealing time of 7 hours.

The same measurements as those carried out in Example 1 were carried outfor the obtained hydrogen absorbing alloy.

Example 21

A hydrogen absorbing alloy with a composition ofLa_(17.0)Mg_(4.1)Ni_(73.9)Mn_(0.9)Al_(4.1) was obtained by carrying outthe same operation as that of Example 3, except that prescribed amountsof raw material ingots were weighed in accordance with the intendedcomposition and recrystallization annealing was carried out at anannealing temperature of 940° C. under 0.2 MPa for an annealing time of7 hours.

The same measurements as those carried out in Example 1 were carried outfor the obtained hydrogen absorbing alloy.

Example 22

A hydrogen absorbing alloy with a composition ofLa_(17.0)Mg_(4.1)Ni_(73.7)Co_(4.7)Mn_(0.2)Al_(0.3) was obtained bycarrying out the same operation as that of Example 3, except thatprescribed amounts of raw material ingots were weighed in accordancewith the intended composition and recrystallization annealing wascarried out at an annealing temperature of 940° C. under 0.2 MPa for anannealing time of 7 hours.

The same measurements as those carried out in Example 1 were carried outfor the obtained hydrogen absorbing alloy.

Comparative Example 1

A hydrogen absorbing alloy with a composition ofLa_(13.1)Ce_(1.8)Nd_(1.1)Ni_(64.1)Co_(9.9)Mn_(5.0)Al_(5.0) was obtainedby carrying out the same operation as that of Example 2, except thatprescribed amounts of raw material ingots were weighed in accordancewith the intended composition and recrystallization annealing wascarried out at an annealing temperature of 100° C. for an annealing timeof 7 hours.

The same measurements as those carried out in Example 1 were carried outfor the obtained hydrogen absorbing alloy.

Comparative Example 2

A hydrogen absorbing alloy with a composition ofLa_(17.1)Mg_(8.0)Ni_(62.2)Co_(12.7) was obtained by carrying out thesame operation as that of Example 2, except that prescribed amounts ofraw material ingots were weighed in accordance with the intendedcomposition and recrystallization annealing was carried out at anannealing temperature of 1000° C. for an annealing time of 7 hours.

The same measurements as those carried out in Example 1 were carried outfor the obtained hydrogen absorbing alloy.

Comparative Example 3

In accordance with an intended composition, prescribed amounts of rawmaterial ingots were weighed, put in a crucible and heated to 1500° C.using a high frequency melting furnace in a helium gas atmosphere atreduced pressure to melt the materials.

After the melting, the melted alloy was solidified by a die castingmethod involving leaving the alloy in a die in the furnace to obtain ahydrogen absorbing alloy with a composition ofLa_(16.9)Mg_(4.1)Ni_(71.1)Co_(5.8)Mn_(1.0)Al_(1.1).

No recrystallization annealing was carried out for the obtained hydrogenabsorbing alloy. The same measurements as those carried out in Example 1were carried out for the obtained hydrogen absorbing alloy.

Comparative Example 4

A hydrogen absorbing alloy with a composition ofLa_(18.2)Mg_(3.0)Ni_(68.3)Co_(6.3)Mn_(2.1)Al_(2.1) was obtained bycarrying out the same operation as that of Example 3, except thatprescribed amounts of raw material ingots were weighed in accordancewith the intended composition and recrystallization annealing wascarried out at an annealing temperature of 940° C. under 0.2 MPa for anannealing time of 7 hours.

The same measurements as those carried out in Example 1 were carried outfor the obtained hydrogen absorbing alloy.

Comparative Example 5

A hydrogen absorbing alloy with a composition ofLa_(14.5)Mg_(6.5)Ni_(69.4)Co_(5.9)Mn_(1.9)A_(1.8) was obtained bycarrying out the same operation as that of Example 3, except thatprescribed amounts of raw material ingots were weighed in accordancewith the intended composition and recrystallization annealing wascarried out at an annealing temperature of 940° C. under 0.2 MPa for anannealing time of 7 hours.

The same measurements as those carried out in Example 1 were carried outfor the obtained hydrogen absorbing alloy.

Comparative Example 6

A hydrogen absorbing alloy with a composition ofLa_(17.8)Mg_(4.3)Ni_(75.6)Mn_(2.3) was obtained by carrying out the sameoperation as that of Example 3, except that prescribed amounts of rawmaterial ingots were weighed in accordance with the intended compositionand recrystallization annealing was carried out at an annealingtemperature of 940° C. under 0.2 MPa for an annealing time of 7 hours.

The same measurements as those carried out in Example 1 were carried outfor the obtained hydrogen absorbing alloy.

Comparative Example 7

A hydrogen absorbing alloy with a composition ofLa_(16.4)Mg_(3.6)Ni_(78.5)Mn_(1.5) was obtained by carrying out the sameoperation as that of Example 3, except that prescribed amounts of rawmaterial ingots were weighed in accordance with the intended compositionand recrystallization annealing was carried out at an annealingtemperature of 940° C. under 0.2 MPa for an annealing time of 7 hours.

The same measurements as those carried out in Example 1 were carried outfor the obtained hydrogen absorbing alloy.

Comparative Example 8

A hydrogen absorbing alloy with a composition ofLa_(16.9)Mg_(4.0)Ni_(72.2)Mn_(1.9)Al_(5.0) was obtained by carrying outthe same operation as that of Example 3, except that prescribed amountsof raw material ingots were weighed in accordance with the intendedcomposition and recrystallization annealing was carried out at anannealing temperature of 940° C. under 0.2 MPa for an annealing time of7 hours.

The same measurements as those carried out in Example 1 were carried outfor the obtained hydrogen absorbing alloy.

Comparative Example 9

A hydrogen absorbing alloy with a composition ofLa_(16.9)Mg_(4.0)Ni_(74.8)Co_(4.1)Al_(0.2) was obtained by carrying outthe same operation as that of Example 3, except that prescribed amountsof raw material ingots were weighed in accordance with the intendedcomposition and recrystallization annealing was carried out at anannealing temperature of 940° C. under 0.2 MPa for an annealing time of7 hours.

The same measurements as those carried out in Example 1 were carried outfor the obtained hydrogen absorbing alloy.

The production conditions and measurement results for theabove-mentioned Examples and Comparative Examples are shown in thefollowing Tables 1 to 4.

TABLE 1 Example Example Example Example Example Example Example Example1 2 3 4 5 6 7 8 Composition B B B B B B B B Production method die diemelt melt melt melt melt melt casting casting spinning spinning spinningspinning spinning spinning Heat treatment pressure Atmospheric 0.2 0.20.2 0.2 0.2 0.2 0.5 condition (MPa) pressure Heat treatment 940° C. 940°C. 860° C. 900° C. 940° C. 980° C. 1020° C. 940° C. temperature and time7 hr 7 hr 7 hr 7 hr 7 hr 7 hr 7 hr 7 hr Rietveld Rwp 3.76 3.70 3.52 3.253.35 3.33 4.06 3.50 analysis Re 1.87 1.90 1.80 1.85 2.01 2.05 1.99 2.01S 2.01 1.95 1.96 1.76 1.67 1.62 2.04 1.74 Crystal AB5(CaCu5) 5 18 17 2213 17 38 5 structure A5B19(Ce5Co19) 20 30 56 10 0 0 14 0 (weight %)A2B7(Ce2Ni7) 34 10 22 24 5 4 36 5 (Gd2Co7) AB3(PuNi3) 0 0 0 0 0 0 0 0A5B19(Pr5Co19) 39 42 5 44 82 79 12 90 AB2(AuBe5) 2 0 0 0 0 0 2 0Discharge capacity at 10th 351 340 345 351 346 343 325 347 cycle (0.2ItA discharge) (mAh/g) Cycle deterioration ratio 93.1 94 95.4 96.8 98.198.4 94.5 98.4 (53rd/10th) (%) Saturated magnetization 4.22 3.9 3.523.46 3.1 2.95 3.55 2.55 per mass after cycle test (Am²/kg) Specificsurface area after 3.75 3.41 2.26 2.22 2.05 2.01 2.61 1.94 cycle testm²/g)

TABLE 2 Example Example Example Example Example Example Example Example9 10 11 12 13 14 15 16 Composition B A C D E F G H Production methodmelt melt melt melt melt melt melt melt spinning spinning spinningspinning spinning spinning spinning spinning Heat treatment pressureAtmospheric 0.2 0.2 0.2 0.2 0.2 0.2 0.2 condition (MPa) pressure Heattreatment 940° C. 940° C. 940° C. 940° C. 940° C. 940° C. 940° C. 940°C. temperature and time 7 hr 7 hr 7 hr 7 hr 7 hr 7 hr 7 hr 7 hr RietveldRwp 3.65 3.24 3.55 4.22 3.99 3.45 3.77 3.35 analysis Re 1.88 1.88 1.902.01 2.11 1.97 1.96 1.97 S 1.94 1.72 1.87 2.10 1.89 1.75 1.92 1.70Crystal AB5(CaCu5) 15 16 20 22 21 20 23 21 structure A5B19(Ce5Co19) 5 04 4 4 10 6 5 (weight %) A2B7(Ce2Ni7) 10 7 9 9 5 15 13 14 (Gd2Co7)AB3(PuNi3) 0 0 0 0 0 0 0 0 A5B19(Pr5Co19) 70 77 67 65 70 55 58 60AB2(AuBe5) 0 0 0 0 0 0 0 0 Discharge capacity at 10th 346 340 337 336335 337 332 335 cycle (0.2 ItA discharge) (mAh/g) Cycle deteriorationratio 96 97.3 96.5 96.4 96.7 95.5 96.2 96.4 (53rd/10th) (%) Saturatedmagnetization per 3.5 3.31 2.76 3.44 3.5 3.55 3.4 2.89 mass after cycletest (Am²/kg) Specific surface area after 2.25 2.21 2.31 2.33 2.39 2.442.3 2.22 cycle test (m²/g)

TABLE 3 Comparative Comparative Example Example Example Example ExampleExample Example Example 17 18 19 20 21 22 1 2 Composition I M N O T U JK Production method melt melt melt melt melt melt die die spinningspinning spinning spinning spinning spinning casting casting Heattreatment pressure 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 condition (MPa) Heattreatment 940° C. 940° C. 940° C. 940° C. 940° C. 940° C. 1000° C. 1000°C. temperature and time 7 hr 7 hr 7 hr 7 hr 7 hr 7 hr 7 hr 7 hr RietveldRwp 3.69 4.12 3.87 4.01 3.54 3.46 3.04 4.59 analysis Re 1.87 2.08 1.982.11 2.11 2.05 1.81 2.12 S 1.97 1.98 1.95 1.9 1.68 1.69 1.68 2.17Crystal AB5(CaCu5) 6 25 11 16 19 17 100 0 structure A5B19(Ce5Co19) 0 2621 14 15 14 0 18 (weight %) A2B7(Ce2Ni7) 19 38 17 37 46 43 0 70 (Gd2Co7)AB3(PuNi3) 0 0 0 0 0 0 0 7 A5B19(Pr5Co19) 75 8 51 31 13 26 0 0AB2(AuBe5) 0 3 0 2 7 0 0 5 Discharge capacity at 10th 360 333 342 340336 341 302 327 cycle (0.2 ItA discharge) (mAh/g) Cycle deteriorationratio 96.9 93.9 95.5 94.8 94.4 94.1 99.3 90 (53rd/10th) (%) Saturatedmagnetization 2.88 3.75 3.55 3.71 3.65 3.6 2.27 7.21 per mass aftercycle test (Am²/kg) Specific surface area after 2.3 3.11 2.99 3.01 3.053.11 0.94 6.95 cycle test (m²/g)

TABLE 4 Comparative Comparative Comparative Comparative ComparativeComparative Comparative Example Example Example Example Example ExampleExample 3 4 5 6 7 8 9 Composition B L P Q R S V Production method diemelt melt melt melt melt melt casting spinning spinning spinningspinning spinning spinning Heat treatment pressure none 0.2 0.2 0.2 0.20.2 0.2 condition (MPa) Heat treatment none 940° C. 940° C. 940° C. 940°C. 940° C. 940° C. temperature and time 7 hr 7 hr 7 hr 7 hr 7 hr 7 hrRietveld Rwp 3.56 4.99 5.12 4.23 3.98 3.88 3.55 analysis Re 1.91 2.142.11 2.01 1.99 1.97 2.05 S 1.86 2.33 2.43 2.10 2.00 1.97 1.73 CrystalAB5(CaCu5) 45 33 29 17 22 15 20 structure A5B19(Ce5Co19) 26 3 7 20 22 2022 (weight %) A2B7(Ce2Ni7) 10 42 50 61 53 49 55 (Gd2Co7) AB3(PuNi3) 0 00 0 0 0 0 A5B19(Pr5Co19) 0 0 0 2 3 0 3 AB2(AuBe5) 7 22 14 0 0 16 0Discharge capacity at 10th 315 319 316 344 339 327 339 cycle (0.2 ItAdischarge) (mAh/g) Cycle deterioration ratio 88.2 92.1 90.1 92.3 92.391.5 91.4 (53rd/10th) (%) Saturated magnetization 6.21 4.56 5.13 4.444.34 4.68 4.66 per mass after cycle test (Am²/kg) Specific surface areaafter 5.5 4.12 4.85 3.95 3.81 4.53 4.32 cycle test (m²/g)

The compositions of the hydrogen absorbing alloys in Tables 1 to 4 areas follows.

A: La_(16.9)Mg_(4.1)Ni_(69.2)Co_(6.0)Mn_(1.9)Al_(1.9), B:La_(16.9)Mg_(4.1)Ni_(71.1)Co_(5.8)Mn_(1.0)Al_(1.1), C:La_(17.0)Mg_(4.2)Ni_(77.0)Mn_(1.8), D:La_(17.2)Mg_(4.0)Ni_(73.3)Co_(3.9)Mn_(1.6), E:La_(12.8)Pr_(4.0)Mg_(3.6)Ni_(72.1)Co_(4.4)Mn_(3.1), F:La_(13.4)Ce_(4.2)Mg_(3.6)Ni_(71.1)Co_(4.0)Mn_(3.7), G:La_(13.9)Ce_(2.1)Nd_(0.8)Mg_(4.1)Ni_(72.2)Co_(4.0)Mn_(2.9), H:La_(14.1)Ce_(2.0)Nd_(0.9)Mg_(4.1)Ni_(76.3)Mn_(2.6), I:La_(17.6)Mg_(3.6)Ni_(76.4)Mn_(1.4)A_(1.0), J:La_(13.1)Ce_(1.8)Nd_(1.1)Ni_(64.1)Co_(9.9)Mn_(5.0)Al_(5.0), K:La_(17.1)Mg_(8.0)Ni_(62.2)Co_(12.7), L:La_(15.2)Mg_(3.0)Ni_(68.3)Co_(6.3)Mn_(2.1)Al_(2.1), M:La_(17.5)Mg_(3.4)Ni_(68.2)Cu_(6.4)Mn_(2.1)Al_(2.1), N:La_(16.3)Mg_(4.7)Ni_(69.9)Co_(6.0)Mn_(2.0)Al_(1.1), O:La_(15.8)Mg_(5.0)Ni_(71.1)Co_(4.1)Mn_(2.0)Al_(2.0), P:La_(14.5)Mg_(6.5)Ni_(69.4)Cu_(5.9)Mn_(1.9)Al_(1.8), Q:La_(17.8)Mg_(1.3)Ni_(75.6)Mn_(2.3), R:La_(16.4)Mg_(3.6)Ni_(78.5)Mn_(1.5), S:La_(16.9)Mg_(4.0)Ni_(72.2)Mn_(1.9)Al_(5.0), T:La_(17.0)Mg_(4.1)Ni_(73.9)Mn_(0.9)Al_(4.1), U:La_(17.0)Mg_(4.1)Ni_(73.7)Cu_(4.7)Mn_(0.2)Al_(0.3), V:La_(16.9)Mg_(4.0)Ni_(74.8)Co_(4.1)Al_(0.2)

As shown in Tables 1 to 4, it was found that the hydrogen absorbingalloys obtained in Examples 1 to 22 had high capacities and wereexcellent in durability to the charging and discharging cycles.

Experiment Example 1

As shown in Table 5, prescribed amounts of raw material ingots havingchemical compositions 1 to 5 with different Cu element contents wereweighed and put in crucibles and heated to 1500° C. using a highfrequency melting furnace in an argon gas atmosphere at reduced pressureto melt the materials. After the melting, the melted alloys weresolidified by quenching by a melt spinning method.

Next, the obtained alloys were heat-treated at respective temperaturesshown in Table 1 in an argon gas atmosphere pressurized to 0.2 MPa(gauge pressure, hereinafter the same). The crystal structure, existenceratio of phases, and average particle size were measured in the samemanner as described above for the obtained alloys. The production ratiosof the Pr₅Co₁₉ type crystal structure phase are shown in Table 5 andFIG. 3.

TABLE 5 Production ratio of Pr₅Co₁₉ type crystal structure phase (weight%) No heat Chemical composition treatment 820° C. 840° C. 860° C. 920°C. 970° C. 980° C. 1030° C. La_(17.0)Mg_(4.3)Ni_(76.6)Mn_(2.1)Composition 1 7 14 15 13 19 22 20 10 (containing no Cu)La_(17.0)Mg_(4.3)Ni_(75.5)Mn_(2.1)Cu_(1.1) Composition 2 0 15 21 37 5243 28 15 La_(17.0)Mg_(4.3)Ni_(74.5)Mn_(2.1)Cu_(2.1) Composition 3 0 1724 35 70 67 40 18 La_(17.0)Mg_(4.3)Ni_(72.3)Mn_(2.1)Cu_(4.3) Composition4 0 15 18 40 81 45 34 15 (standard)La_(17.0)Mg_(4.3)Ni_(70.2)Mn_(2.1)Cu_(6.4) Composition 5 5 20 30 39 6538 28 12

In the above-mentioned Experiment Example, the charging and dischargingcharacteristics measurement was also carried out in the same manner asdescribed above.

The results are shown in Table 6 and FIG. 4.

TABLE 6 Results of capacity retention ratio (%) No heat treatment 820°C. 840° C. 860° C. 920° C. 970° C. 980° C. 1030° C. Composition 1 68 7277 76 76 77 75 74 Composition 2 70 73 77 80 81 79 77 74 Composition 3 7277 78 80 85 86 79 76 Composition 4 71 76 78 82 87 85 80 75 Composition 572 75 79 81 81 79 78 74

Experiment Example 2

The respective experiments and measurements were carried out in the samemanner as in Experiment Example 1, except that as shown in Table 7, rawmaterial ingots having chemical compositions 1, 4, and 6 to 8 withdifferent Mn element contents and containing an Al element in place ofthe Mn element were used. The production ratios of the phase of thePr₅Co₁₉ type crystal structure are shown in Table 7 and FIG. 5 and themeasurement results of the capacity retention ratios are shown in Table8 and FIG. 6.

TABLE 7 Production ratio of Pr₅Co₁₉ type crystal structure phase (weight%) No heat Chemical composition treatment 820° C. 840° C. 860° C. 920°C. 970° C. 980° C. 1030° C. La_(17.0)Mg_(4.3)Ni_(76.6)Mn_(2.1)Composition 1 7 14 15 13 19 22 20 10 (containing no Cu)La_(17.0)Mg_(4.3)Ni_(72.3)Mn_(2.1)Cu_(4.3) Composition 4 0 15 18 40 8145 34 15 (standard) La_(17.0)Mg_(4.3)Ni_(70.2)Mn_(4.3)Cu_(4.3)Composition 6 5 20 28 43 75 35 25 8La_(17.0)Mg_(4.3)Ni_(68.1)Mn_(6.4)Cu_(4.3) Composition 7 0 23 35 45 6834 20 5 La_(17.0)Mg_(4.3)Ni_(70.2)Al_(4.3)Cu_(4.3) Composition 8 0 18 2540 70 30 15 3

TABLE 8 Results of capacity retention ratio (%) No heat treatment 820°C. 840° C. 860° C. 920° C. 970° C. 980° C. 1030° C. Composition 1 68 7277 76 76 77 75 74 (containing no Cu) Composition 4 71 76 78 82 87 85 8075 (standard) Composition 6 69 75 78 82 84 83 78 76 Composition 7 68 7277 80 80 79 76 70 Composition 8 69 74 79 80 85 78 72 69

Experiment Example 3

The respective experiments and measurements were carried out in the samemanner as in Experiment Example 1, except that as shown in Table 9, rawmaterial ingots having chemical compositions with different contents oftotal amounts of La and Mn elements were used. The production ratios ofthe phase of the Pr₅Co₁₉ type crystal structure are shown in Table 9 andFIG. 7 and the measurement results of the capacity retention ratios areshown in Table 10 and FIG. 8.

TABLE 9 Production ratio of Pr₅Co₁₉ type crystal structure phase (weight%) No heat Chemical composition treatment 820° C. 840° C. 860° C. 920°C. 970° C. 980° C. 1030° C. La_(17.0)Mg_(4.3)Ni_(76.6)Mn_(2.1)Composition 1 7 14 15 13 19 22 20 10 (containing no Cu)La_(19.1)Mg_(2.1)Ni_(72.3)Mn_(2.1)Cu_(4.3) Composition 9 4 12 13 42 5550 38 10 La_(17.0)Mg_(4.3)Ni_(72.3)Mn_(2.1)Cu_(4.3) Composition 4 0 1518 40 81 45 34 15 (standard) La_(14.9)Mg_(6.4)Ni_(72.3)Mn_(2.1)Cu_(4.3)Composition 10 4 20 25 25 70 38 21 12

TABLE 10 Results of capacity retention ratio (%) No heat treatment 820°C. 840° C. 860° C. 920° C. 970° C. 980° C. 1030° C. Composition 1 68 7277 76 76 77 75 74 (containing no Cu) Composition 9 73 74 80 81 86 87 8277 Composition 4 71 76 78 82 87 85 80 75 (standard) Composition 10 68 7378 81 83 83 77 74

Experiment Example 4

The respective experiments and measurements were carried out in the samemanner as in Experiment Example 1, except that as shown in Table 11, rawmaterial ingots having chemical compositions with different contents ofan Ni element were used. The production ratios of the phase of thePr₅Co₁₉ type crystal structure are shown in Table 11 and FIG. 9 and themeasurement results of the capacity retention ratios are shown in Table12 and FIG. 10.

TABLE 11 Production ratio of Pr₅Co₁₉ type crystal structure phase(weight %) No heat Chemical composition treatment 820° C. 840° C. 860°C. 920° C. 970° C. 980° C. 1030° C. La_(17.0)Mg_(4.3)Ni_(76.6)Mn_(2.1)Composition 1 7 14 15 13 19 22 20 10 (containing no Cu)La_(17.8)Mg_(4.4)Ni_(71.1)Mn_(2.2)Cu_(4.4) Composition 11 6 13 18 30 5648 28 8 La_(17.4)Mg_(4.3)Ni_(71.7)Mn_(2.2)Cu_(4.3) Composition 12 4 1213 35 60 50 30 8 La_(17.0)Mg_(4.3)Ni_(72.3)Mn_(2.1)Cu_(4.3) Composition4 0 15 18 40 81 45 34 15 (standard)La_(16.7)Mg_(4.2)Ni_(72.9)Mn_(2.1)Cu_(4.3) Composition 13 4 20 25 38 7038 30 8

TABLE 12 Results of capacity retention ratio (5) No heat treatment 820°C. 840° C. 860° C. 920° C. 970° C. 980° C. 1030° C. Composition 1 68 7277 76 76 77 75 74 (containing no Cu) Composition 11 68 72 75 78 80 80 7873 Composition 12 70 74 76 81 84 84 79 75 Composition 4 71 76 78 82 8785 80 75 (standard) Composition 13 73 78 78 81 86 85 78 75

1. A hydrogen absorbing alloy containing a phase of a Pr₅Co₁₉ typecrystal structure having a composition defined by a general formulaA_((4−w))B_((1+w))C₁₉ (A denotes one or more element(s) selected fromrare earth elements including Y (yttrium); B denotes an Mg element; Cdenotes one or more element(s) selected from a group consisting of Ni,Co, Mn, and Al; and w denotes a numeral in a range from −0.1 to 0.8) andhaving a composition as a whole defined by a general formulaR1_(x)R2_(y)R3_(z) (15.8≦x≦17.8, 3.4≦y≦5.0, 78.8≦z≦79.6, and x+y+z=100;R1 denotes one or more element(s) selected from rare earth elementsincluding Y (yttrium); R2 denotes an Mg element, R3 denotes one or moreelement(s) selected from a group consisting of Ni, Co, Mn, and Al; thenumeral of Mn+Al in said z is 0.5 or higher; and the numeral of Al insaid z is 4.1 or lower).
 2. The hydrogen absorbing alloy according toclaim 1, wherein in said general formula R1_(x)R2_(y)R3_(z), said x, y,and z satisfy 16.3≦x≦17.6, 3.6≦y≦4.7, and 78.8≦z≦79.1; the numeral ofMn+Al in said z is 1.6 or higher; and the numeral of Al in said z is 1.9or lower.
 3. The hydrogen absorbing alloy according to claim 1, whereinthe phase of said Pr₅Co₁₉ type crystal structure is formed at a ratio of8 weight % or more.
 4. A hydrogen absorbing alloy containing 15 weight %or higher of a phase of a Pr₅Co₁₉ type crystal structure and obtained bymelting and annealing in the state that the content of a Cu element iscontrolled to be 1 to 8 mol %.
 5. The hydrogen absorbing alloy accordingto claim 4, wherein the alloy as a whole has a composition defined by ageneral formula R1_(a)R2_(b)R3_(c)Cu_(d) (R1 denotes one or moreelement(s) selected from rare earth elements including Y (yttrium); R2denotes one or more element(s) selected from a group consisting of Mg,Ca, Sr, and Ba; R3 denotes one or more element(s) selected from a groupconsisting of Ni, Co, Mn, Al, Fe, Cr, Zn, Si, Sn, V, Nb, Ta, Ti, Zr, andHf; and a, b, c, and d denote numerals satisfying 15≦a≦19, 2≦b≦7,70≦c≦80, 1≦d≦7, and a+b+c+d=100, respectively).
 6. The hydrogenabsorbing alloy according to claim 5, wherein said R2 is Mg and said R3is one or more element(s) selected from a group consisting of Co, Mn,Al, and Ni.
 7. The hydrogen absorbing alloy according to claim 1,wherein the alloy has a primary grain size of 10 to 100 nm.
 8. Ahydrogen absorbing alloy production method for producing the hydrogenabsorbing alloy according to claim 1, wherein the method comprises stepsof cooling a melted alloy at a cooling speed of 1000 K/second or moreand further annealing the obtained alloy at a temperature in a rangefrom 860 to 1020° C. in an inert gas atmosphere under a pressurizedstate.
 9. A hydrogen absorbing alloy production method for producing thehydrogen absorbing alloy according to claim 4, wherein the methodcomprises steps of cooling a melted alloy at a cooling speed of 1000K/second or more and further annealing the obtained alloy at atemperature in a range from 860 to 980° C. in an inert gas atmosphereunder a pressurized state.
 10. The hydrogen absorbing alloy productionmethod according to claim 9, wherein the temperature range of saidannealing is in a range from 920 to 970° C.
 11. A hydrogen absorbingalloy electrode using the hydrogen absorbing alloy according to claim 1as a hydrogen storage medium.
 12. A secondary battery provided with thehydrogen absorbing alloy electrode according to claim 11.